Remove Py3.9 testing: It's new and weird

https://github.com/kernc/backtesting.py/runs/1318477083

Author: kernc

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.9]
+        python-version: [3.6, 3.7]
         include:
           - python-version: 3.8
             test-type: lint